I'd still carry something in the car. Your knife works, you're right. It doesn't work and your not around anymore? I would think after I tried my knife and said "Oh crap, that didn't work, what else can I do"? that you would then remember where you placed it. As far as placing it there 2 years ago, like all SAFETY equipment, you should check it at specified intervals to make sure that it is functioning properly. The more you check it, the more you will remember its there. And remember, safety in redundancy....

Ultimately it's your call. Knowing ahead of time and not carrying something when around water, for some people that would be like letting someone who has had too much to drink give you a ride home.

my .02